Current US-European divisions over the precautionary principle parallel historical gaps between the common law and civil law over the burden of proof needed for state intervention into markets. Yet research into cross-national variation in environmental policy has mostly neglected legal traditions, due partly to the difficulty of reconciling this thesis with evident regulatory variation across common law countries. This paper responds to this challenge by analyzing continental influences on mid-19th-century British public health and environmental reforms, and the significance of common law principles in the claims of their opponents. In particular the paper focuses on the regulatory regime established under the Alkali Acts of 1863, and 1876. The Act, which required technology-based reductions in specified "noxious vapours" from chemical works irrespective of proven harm to health, has correctly been recognized as the harbinger of precautionary regulation in Britain. Historical explanations of this development have focused entirely on domestic factors, entrenching in the process an understanding of the Alkali Act - and Britain more generally - as the origin of centralized precautionary environmental regulation. By contrast, this paper relies on overlooked primary documents to argue that the Alkali Act was directly inspired by French and other continental regulatory models and that the regime it ultimately created constituted a hybrid of continental and common law regulatory principles. Once we understand the Alkali Act regime as partially transplanted from the continent, it becomes easier to reconcile an understanding of precautionary regulation as a civil law instrument with the evident presence of that instrument in Victorian Britain. This in turn, should facilitate greater attention to the influence of legal traditions in studies of national legal styles and comparative environmental politics more generally.